The disclosure of Japanese Patent Application No. 2001-374828 filed on Dec. 7, 2001 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a device for purification of exhaust gas of an internal combustion engine, and in particular, relates to an exhaust gas purification device having means for removal of nitrogen oxides (NOx) contained within the exhaust gas.
2. Description of the Related Art
Generally, direct injection internal combustion engines, for example, diesel engines, are required to remove exhaust particulate matter such as soot, as well as nitrogen oxides (NOx), contained within exhaust gas. In order to address this requirement, a method has been proposed in which a particulate filter supported by a NOx absorbent is disposed in an exhaust gas passage of an internal combustion engine.
The NOx absorbent used in such cases absorbs NOx when the air-fuel ratio is lean. In addition, the NOx absorbed in the NOx absorbent is reduced and purified, if the air-fuel ratio of the exhaust gas becomes rich and there is a reductant such as HC or CO present within the exhaust gas also (by a NOx absorption-discharge-reduction action or a NOx absorption-reduction action). By utilizing this action, NOx within the exhaust gas is absorbed into the NOx absorbent when the air-fuel ratio of the exhaust gas is lean. However, after use over a given period of time, when the absorption efficiency of the NOx absorbent falls or just prior to absorption efficiency falling, the reductant (fuel) is supplied, or the like, to the NOx absorbent, and reduction-purification of the absorbed NOx is executed. It should be noted that in this specification the term xe2x80x9cabsorptionxe2x80x9d also includes the meaning xe2x80x9cadsorptionxe2x80x9d. Accordingly, the term xe2x80x9cNOx absorbentxe2x80x9d which will be used hereinafter, includes both a xe2x80x9cNOx absorption agentxe2x80x9d that stores NOx in the form of nitrate, or the like, and a xe2x80x9cNOx adsorption agentxe2x80x9d that adsorbs NOx as NO2, and the like.
However, there are cases when the fuel of the internal combustion engine contains sulfur (S) components, and as a result, sulfur oxides (SOx) are contained within the exhaust gas. When SOx is present within the exhaust gas, the NOx absorbent executes the absorption of SOx using exactly the same mechanism as is used to execute the absorption of NOx within the exhaust gas.
However, the SOx which is absorbed into the NOx absorbent is comparatively stable, and is, generally speaking, easily stored in the NOx absorbent. If the amount of SOx stored in the NOx absorbent increases, the NOx storage amount of the NOx absorbent decreases. As a result, the NOx absorbent becomes unable to adequately remove NOx within the exhaust gas and thus the problem of so-called sulfur poisoning (hereinafter referred to as xe2x80x9cS poisoning), in other words, a fall in NOx purification efficiency occurs. In particular, the problem of S poisoning has a tendency to occur in diesel engines that use diesel oils which contain a comparatively large sulfur component within the fuel.
On the other hand, it is widely known that SOx stored within the NOx absorption agent can be discharged, desorped, and the like, by the same mechanism as for NOx. However, since SOx is stored in the NOx absorbent in a comparatively stable form, it is difficult for the stored SOx to be discharged at the temperature at which the NOx reduction-purification control is executed (for example, at around 250xc2x0 C. or above). Accordingly, in order to solve the problem of S poisoning it is necessary to raise the temperature of the NOx absorbent to a temperature higher than the normal temperature for executing NOx reduction-purification control, on in other words, to a temperature at which sulfur is discharged (for example, 600xc2x0 C. or above). Furthermore, it is also necessary to change the air-fuel ratio of the inflowing exhaust gas to a substantially stoichiometric condition or a rich condition (hereinafter these conditions are simply referred to as a xe2x80x9crich conditionxe2x80x9d) and periodically execute S poisoning regeneration control.
It is presumed that a temperature condition at which S poisoning regeneration control can be executed is obtainable at times of high load/high engine rpm combustion engine becomes high. However, when the internal combustion engine is in such a high load/high rpm operational state, the amount of exhaust gas discharged from the internal combustion engine increases. As a result, in order to form an exhaust gas with a rich air-fuel ratio for execution of the S poisoning regeneration control, a large amount of fuel (reductant) appropriate to the amount of the exhaust gas is necessary. This, however, leads to a deterioration in fuel consumption. Moreover, in this case, the flow rate of the exhaust gas flowing through the NOx absorbent becomes higher, and thus the contact time of the exhaust gas and the NOx absorbent becomes shorter. As a result, sufficient reaction time for the reductant contained in the fuel can no longer be obtained, and problems related to worsening emissions occur (for example, the amount of hydrocarbons that elude the absorption agent increases).
In order to suppress such deterioration in fuel consumption and worsening emissions, a method for executing the S poisoning regeneration control has been disclosed. In this method, in contrast to above, the temperature of the NOx absorbent is raised by adding fuel to the exhaust gas at times such as idling stop and deceleration when the exhaust gas amount discharged from the internal combustion engine becomes less. However, in this case, the amount of exhaust gas discharged from the internal combustion engine is small and thus the heating value of the combustion of the added fuel is limited. Accordingly, this method suffers from the problem that it takes time for the temperature of the NOx absorbent to rise.
In addition, a heating method using heating means such as an electric heater or a burner, is known as a method for raising the temperature of the NOx absorbent. However, in this case, problems arise such as an increase in device cost due to providing the heating means, and an increase in fuel consumption resulting from the energy required for heating.
In the light of the foregoing circumstances, it is an object of the invention to provide an exhaust gas purification device which can, with respect to S poisoning regeneration of a NOx absorbent, reduce regeneration time and suppress the worsening of emissions and deterioration in fuel consumption.
In order to accomplish the above object, an exhaust gas purification device according to a first aspect of the invention is provided with a NOx absorbent, an exhaust gas flow rate control portion, a reductant addition portion, a temperature rise control portion, a rich condition control portion, and an S poisoning regeneration control portion. The NOx absorbent is disposed in an exhaust gas passage through which exhaust gas discharged from an internal combustion engine passes, and absorbs NOx when the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification device is lean, and reduces and purifies the absorbed NOx when the air-fuel ratio of the inflowing exhaust gas becomes richer. The exhaust gas flow rate control portion controls a flow rate of the exhaust gas flowing to the NOx absorbent. The reductant addition portion adds reductant into the exhaust gas passage upstream of the NOx absorbent. The temperature rise control portion controls a temperature of the NOx absorbent such that the temperature is higher than a predetermined temperature; a rich condition control portion for controlling the air-fuel ratio of the exhaust gas flowing to the NOx absorbent such that a substantially stoichiometric condition or a rich condition is formed. The S poisoning regeneration control portion for discharging sulfur components from the NOx absorbing portion executes control of the temperature rise control portion and the rich condition control portion in succession, and furthermore, executes a control of the exhaust gas flow rate control portion such that the flow rate of the exhaust gas flowing to the NOx absorbent is less during execution of control of the rich condition control portion than during execution of control of the temperature rise control portion.
Furthermore, according to the first aspect, the exhaust gas purification device may further include a by-pass passage that by-passes the NOx absorbent. Moreover, the exhaust gas flow rate control portion may be configured such that it can control both the flow rate of the exhaust gas flowing to the NOx absorbent, and the flow rate of the exhaust gas which by-passes the NOx absorbent.